1. Field of the Invention
The present invention relates to an ultrasonic diagnostic apparatus which transmits ultrasonic waves to a moving target such as blood flow and detects frequency shifts (Doppler shifts) of reflected ultrasonic waves from the blood flow to detect the flow direction and velocity.
2. Description of the Related Art
The ultrasonic diagnostic apparatus is used in the medical field to provide information in regard to a moving substance, e.g., a cardiovascular blood flow or moving tissue. The ultrasonic wave transmitted into a patient undergoes a frequency shift when the wave is reflected by a moving substance such as blood flow due to the Doppler effect. The velocity of the blood flow can be non-invasively observed by detecting a Doppler signal indicative of the frequency difference between transmitted and received ultrasonic wave.
In the ultrasonic diagnostic apparatus, a burst wave in the form of a series of ultrasonic pulses is used to detect the Doppler signal, since the burst wave has a better signal-to-noise ratio than the single ultrasonic wave. The frequency band of the burst wave is narrower than the single ultrasonic pulse.
In the case of using a burst wave to detect the Doppler signals, the Doppler information of the moving target, such as blood flow velocity, is obtained as follows. As shown in FIG. 3(a), a conventional ultrasonic diagnostic apparatus provides a two dimensional echographic image (2D image, tomographic image) of a patient and a sample volume marker (to be referred to as a marker) indicating observation region on a TV-monitor. An operator adjusts the location and length of the marker to the region of interest (ROI) such as blood vessel on the TV-monitor. Then, the burst wave is repeatedly transmitted from the ultrasound probe into a moving substance of the patient. Echo signals of the burst wave reflected from a moving substance of the patient are received by the ultrasonic probe. The received echo signals are processed to detect the Doppler signal by a conventional method such as quadrature phase detection. Then, the burst wave is repeatedly transmitted and received for the same direction several times. Thus, the Doppler signals can be acquired from the echo signal. The Doppler signals are gated by a range gate circuit in correspondence with the observation range. Only Doppler signals corresponding to the observation range are obtained. A low pass filter obtains the needed Doppler shift frequency by rejecting unnecessary low frequency component of the Doppler signals such as due to a slow movement in a patient. The operation circuit performs processing such as by means of the Fast Fourier Transform (FFT), on the Doppler signals. Namely, the operation circuit obtains the relation between frequency and amplitude to calculate some sampling data (the Doppler signals). The frequency spectrum is converted into luminance by a digital scan converter (DSC), and is provided to a TV-monitor. On the TV-monitor, the relation between the Doppler shift frequency and (observation) time is displayed on the basis of the frequency spectrum. The velocity of blood flow is obtained by the Doppler shift frequency since the velocity of blood flow corresponds to the Doppler shift frequency.
However, the conventional ultrasonic apparatus has a problem next described referring to FIG. 4. FIG. 4 is a timing diagram illustrating the timing and the gate width of the gating of the range gate applied to the Doppler signals when a burst wave is transmitted once. The axis of abscissa shows time corresponding to the distance (depth) in the direction of depth from an extremity of the probe which is at a depth of zero. Signals p1-pn show in order of depth each phase boundary of acoustic impedance (scatter at each depth) on which the ultrasonic pulses reflect, starting from the shallowest position. As shown in FIG. 4, the conventional ultrasonic diagnostic apparatus gates the range gate for the range corresponding to the observation range (the marker) on the TV-monitor. However, the reflected echoes from each depth continue during the burst wave length when the reflected echoes are received, since the burst wave has a predetermined burst wave length. Therefore, it is impossible to obtain reflected echoes which correspond only to the gate width set in the range gate circuit. Namely, not only needed echoes but also unnecessary echoes (p3, p4 and p5 in FIG. 4) are gated. Thus, needed Doppler signals are mixed with the unnecessary Doppler signals. Therefore, the precision in distance about the sample volume in the marker is deteriorated by the mixed echoes. As a result, artifacts are generated in the frequency spectrum.
The conventional ultrasonic diagnostic apparatus uses a gate width corresponding to the space length of the marker, and varies the gate width according to variation of the space length of the marker. But under that condition, the signal-to-noise ratio is not always very good.